It seems like this study doesn't say anything about ADHD or Adderall. All they gave out was caffeine or placebo and they either told the participants they were getting caffeine or Adderall. So all the study really concluded is that if (a neurotypical) college student was fooled into thinking they took Adderall where they really took caffeine they'd perform better than someone who just took caffeine. Which just seems like proving the placebo effect.